You guys must be expecting to make a huge profit on that first house if you're only putting $18k down but expect to have a second property within a year based solely on profits from the first property. And your friends are probably seriously underestimating costs associated with maintenance on said house, even if it's brand new.
You need to ask them to breakdown their analysis of estimated occupation time across all units, rent income per unit, costs associated with maintaining each unit, how they plan on maintaining the property as well as the house itself, if they plan on renting to section 8/other subsidization programs, if they plan on paying some or all of certain utilities in each unit, etc etc etc.
Most people severely underestimate costs associated with a rental property and severely overestimate overall profit. Also, the more units in the house the more problematic things become because people just don't get along with each other and you have to worry about that many more different sets of utilities (boilers, water heaters, electric). And if it's a college town and the plan is to rent to college students that's a whole other ball of wax (which can be very profitable, but can cause severe devastation of your property as well).